Low power consumption and high integration level have become the mainstream for future CMOS integrated circuits design, so as to meet the energy saving and small dimension requirements for portable electronic products, wireless sensing network nodes and biochips. The power consumption, the accuracy and the integration level of a clock generator, which serves as an important component of the digital circuits, even of some analog circuits, has been gaining more and more attention.
Crystal oscillator is an oscillator having high precision and high stability. However, it cannot be integrated into the chip, thus increasing application cost and requiring more space. In recent years, with the development of the semiconductor process, on-chip integration of the accurate clock source under standard CMOS process has draw the industry and the academia's attention. Currently, the commonly used on-chip integration of the clock source can be divided into three categories, namely, on-chip integration of LC oscillator, on-chip integration of RC oscillator and on-chip integration of ring oscillator, respectively.
The accuracy and the phase noise performance of the LC oscillator can parallel that of the crystal oscillator. However, since large inductance cannot be fabricated under the standard CMOS process, additional high speed frequency divider is required in order to obtain low oscillation frequency. Thus, the power consumption cannot be achieved less than 100 uW.
The power consumption of RC oscillator is low, and it is suitable for low frequency applications. However, its main problem is that the process discreteness of the on-chip resistors and capacitors fabricated under CMOS process is large, and the output frequency of the RC oscillator will be affected greatly by the process variation, and the frequency variation caused by the process discreteness can reach up to 50% under typical CMOS process.
The ring oscillator can be used for high frequency applications. The accuracy of ring oscillator can be increased significantly via a specific design. It is shown by the simulation data that, in the standard CMOS process, the frequency of ring oscillator having specific structure is less affected by the process variation. When the bias resistor for producing bias current is disposed outside the chip, the variation of the output frequency of ring oscillator introduced by the process variation can be within 2%. A current controlled ring oscillator has been disclosed in a Chinese patent publication No. CN1669221A. The variation of output frequency under different process, voltage, temperature (PVT) conditions can be less than 10%. However, the frequency of such oscillator is high, and the power consumption is large.
Chinese patent publication No. CN1135118A has disclosed a ring oscillator having frequency control ring, for setting the bias current of the on-chip ring oscillator until the frequency of the ring oscillator is consistent with the target frequency. Although accurate oscillation frequency can be obtained by such off-chip adjustment method, but the method for setting the current by using current compensation method is rather complicated, and by using off-chip adjustment, each chip should be additionally adjusted once after manufacturing, causing the cost being increased.